The present invention relates to an adhesive polyester film. More specifically, it relates to an adhesive polyester film which has excellent adhesion particularly to a sublimable ink layer due to a specific coating layer formed thereon and is therefore suitably used for a sublimation-type thermosensitive image transfer recording medium.
A polyester film has been heretofore used in many fields due to its excellent properties such as mechanical properties, dimensional stability and heat resistance. One of its application fields is a base film for a heat sensitive transfer recording material.
Of heat sensitive transfer recording systems, the use of a sublimation-type thermosensitive image transfer recording system has been increasing rapidly as a recording system capable of outputting a high-quality full color image with ease. The sublimation-type thermosensitive image transfer recording system comprises heating a binder containing a thermal sublimable dye to sublime only a dye, which leaves the binder, and having the image receiving layer of a transferred paper absorb the dye to form a gradation image. In this system, high adhesion between the binder and the base film is required to sublime only the dye and there must not be any reduction in adhesion, which is caused by environmental changes or the elapse of time. If the adhesion is not enough, the binder layer is transferred to the transferred paper, thereby greatly impairing gradation and causing over-transfer.
In general, a polyester film has poor adhesion because of high crystal orientation and is not firmly bonded even when an ink layer is directly formed thereon. Therefore, to improve its adhesion to an ink layer, there is already known a method for applying a physical or chemical treatment to the surface of the film. This method, however, fails to provide sufficiently high adhesion.
A method for treating a film with a primer is widely used because it is very advantageous from the viewpoint of performance and processing. For example, there is known a method for treating a film with a primer such as a polyester resin, acrylic resin, urethane resin, or a mixture of these (see JP-A 8-104064), or a primer prepared by adding a crosslinking agent to the mixture (see JP-A 8-11447 and JP-A 9-175046).
The base film is required not only to be as thin as possible but also to have high strength. It is also required to have a small dimensional change when shrunk by heat at the time of printing.
JP-A 1-171988 discloses that a biaxially oriented polyester film for a sublimation-type thermosensitive image transfer recording material is obtained by applying a coating solution, which contains a reaction product obtained by polymerizing a compound having both a specific substituent and a polymerizable carbon-carbon double bond in an aqueous solution or water dispersion of a polyester, to at least one side of a polyester film and stretching the resulting film. Stated more specifically, the biaxially oriented polyester film is obtained by applying the coating solution to a monoaxially oriented film which has been stretched to 2 to 6 times at 60 to 130xc2x0 C., stretching the coated film to 2 to 6 times in a direction perpendicular to the previous direction at 80 to 130xc2x0 C., and heating the film at 150 to 250xc2x0 C. for 1 to 600 seconds.
JP-A 3-67695 discloses a polyethylene terephthalate film or polyethylene naphthalate film having an adhesive layer, which has been simultaneously stretched and heated, as a base film for a thermal transfer sheet. As for the above simultaneous stretching and heating conditions, examples disclosed in this publication teach that the film is stretched to 4 times in a longitudinal direction at 80xc2x0 C. and then to 4 times in a transverse direction at 110xc2x0 C. and heated at 210xc2x0 C.
JP-A 3-106691 discloses a biaxially oriented polyester film having an adhesive layer as a base film for a thermal transfer sheet. The publication discloses no specific conditions for biaxial stretching.
The above prior arts have the following problems.
A coating film of an ordinary polyester, acrylic or urethane resin or of a mixture of a polyester resin and an acrylic resin is inferior in moisture resistance, water resistance, solvent resistance and blocking resistance although the coating film provides desired adhesion. Although desired adhesion, moisture resistance, water resistance, solvent resistance and blocking resistance can be provided by adding a general crosslinking agent to a mixture of a polyester resin and an acrylic resin, the pot life of a coating in a production process becomes short and it is difficult to handle it. For example, because a melamine-based crosslinking agent has very high reactivity, a crosslinking reaction proceeds at normal temperature, thereby shortening the pot life of a coating and making it difficult to control the quality of the coating solution in a production process. There is also another problem with regard to the dyeing affinity of a sublimable dye for an adhesive layer. Efforts have recently been made to increase the speed of printing. This requires that a higher temperature be applied momentarily. Therefore, an adhesive layer must have heat resistance and adhesion to an ink layer at the time of heating. If adhesion is low, color density and gradation become insufficient. An increase in printing speed increases the quantity of heat received by a transfer recording material, which causes such problems as the large deformation of a film used as a base material, an unclear image, a wrinkled film or, in an extreme case, a printing failure.
It is an object of the present invention to provide an adhesive polyester film.
It is another object of the present invention to provide an adhesive polyester film suitable for use as a base film for a sublimation-type thermosensitive image transfer recording medium.
It is still another object of the present invention to provide an adhesive polyester film suitable for use as a base film for a sublimation-type thermosensitive image transfer recording medium, which has excellent adhesion to a sublimable ink layer, which suppresses the dyeing affinity of a sublimable dye for an adhesive layer, which has small deformation when heated and which provides a transfer image having excellent gradation.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, the above objects and advantages of the present invention are attained by an adhesive polyester film comprising:
(a) a biaxially oriented polyester film base layer which has no point at which the gradient of its temperature-dimensional change curve changes from a positive value to a negative value when temperature is elevated from the glass transition temperature of the polyester to 240xc2x0 C. and which does not show a dimensional change of more than 5%, and
(b) an adhesive layer formed on at least one side of the base layer and comprising a resin selected from the group consisting of:
(i) a water-soluble or water-dispersible resin of a polyester modified by a vinyl resin, and
(ii) a crosslinked resin of a mixture of a water-soluble or water-dispersible acrylic resin, a water-soluble or water-dispersible polyester resin and an epoxy resin crosslinking agent.